1. Field of the Invention
The present invention relates to upper and lower relays and a network system, and in particular to a network system and relays (upper and lower relaying devices) composing the network system, especially a device such as a LAN switch performing a layer 2 switching.
Recently, a spread of the Internet, an expansion of an enterprise IP (Internet Protocol) network (intranet), a provision of an IP virtual leased line (i.e. IP-VPN (IP Virtual Private Network) service by carriers (Type I carriers) and an ISP (Internet Service Provider), and the like accompanied by a rapid spread of a personal computer have made IP data communication more and more important.
Also, together with functions and performances of a network itself enhanced, various services such as e-mail, WWW (World Wide Web), integration of voice and data over IP (i.e. VoIP (Voice over IP)), and movie/voice reproduction have been spreading, whereby the total volume of data traffic has been increasing.
Correspondingly, a network itself has become broadband (high-speed), and multimedia data of large capacity has come into widespread use. In such a network, its reliability is critical.
2. Description of the Related Art
FIG. 21 shows a basic arrangement of a prior art network which offers such services. This network is layered into a backbone network 120 of a carrier, an access network 170, and a network 180 composed of general user devices and enterprise user devices accessing the Internet or the like through the access network 170 and the backbone network 120.
The router backbone network 120 is composed of IP switches (or routers) 100_1–100_4, . . . (hereinafter, occasionally represented by a reference numeral 100) connected with links 110_1, 110_3, 110_4, . . . .
In the backbone network 120 where traffic concentrates, a SONET (Synchronous Optical Network) ring or the like, which is technologically expensive but has a function of detouring a path when a fault or failure occurs in a part of the network, is generally introduced.
The access network 170 is composed of aggregation LAN switches connected to the IP switch 100, and LAN switches connected to the aggregation LAN switches. For example, aggregation LAN switches 140, 140_2, and 140_3 (switches 140_2 and 140_3 are not shown) are respectively connected to the IP switch 100_1 with links 130_1–130_3. LAN switches 160_1–160_8 are respectively connected to the aggregation LAN switch 140_1 with links 150_1–150_8.
Enterprise users or general users are connected to each LAN switch. Enterprise users 191_1, . . . are connected to e.g. the LAN switch 160_1 respectively with links 181_1, . . . . General users 192_1–192_q are connected to the LAN switch 160_2 respectively with links 182_1–182_q.
Similarly, an enterprise user (branch) 194 is connected to the IP switch 100_2 through an aggregation LAN switch 141 and a LAN switch 161.
Thus, the access network 170 and the user network 180 are generally supported by an optical Ethernet (R) where a star-type topology is adopted.
Also, the networks 170 and 180 are generally composed of a switched media method LAN switch or the like, and can inexpensively offer a high-speed communication service and an Internet access service by using existing protocols and technologies such as an IP and Ethernet (R).
Specifically, the network 180 is called a broadband access network which includes the followings: (1) FTTH (Fiber To The Home) connecting a user to an IP network of a carrier with a LAN switch; (2) ADSL (Asymmetrical Digital Subscriber Line) using copper wires which have been used for telephone lines; (3) Wireless LAN, and the like.
Among these, the FTTH is expected in the future, and a construction of an FTTH network connecting end users such as homes and enterprises with optical fibers has already begun.
As described above, the Ethernet (R) technology which has spread within an enterprise network begins to be used for an Internet access or a WAN technology connecting enterprises. However, since the Ethernet (R) technology is a simple point-to-point communication technology, it is inexpensive but basically has no mechanism concerning reliability such as a path detour upon a fault, resulting in a low reliability.
Specifically, the enterprise users, as shown in FIG. 21, connect the head office 191_1 and its branch (e.g. branch 194), or local sites with a VPN service offered by the carriers. Supposing that a fault occurs in a part of the VPN, there is a possibility that communication becomes impossible, which leads to interferences of business operations.
Also, it becomes impossible for the general users to access the Internet when a fault occurs. These lead to a large problem concerning a credibility for the carriers.
While one may conceive to adopt a SONET device or the like having a high reliability for the access network, it is very expensive compared with the LAN switch.
As for the technology to make the Ethernet (R) highly reliable, there is a link aggregation connecting devices with a plurality of Ethernet (R) transmission lines (links).
When a link aggregation technology is adopted for the access network 170 for example, the carriers have to construct the same fibers as the links 150_1–150_8 between an aggregation LAN switch 140 and the LAN switches 160_1–160_8. The constructed fibers become longer as the number of the LAN switches 160 becomes larger.